The present invention relates to a room or other area-security system wherein one or more transmitting elements and one or more receiving elements are relied upon for detection of intruder presence or motion within the security area.
Area-security systems are known to operate on the basis of different physical principles. Thus, it is known from British Pat. No. 715,444 to monitor an area by using a rotating mirror to move a beam of light over a light-reflecting strip which is arranged along one side of the surface to be monitored. From this strip, the beam of light is passed to a second reflection strip, consisting of triple mirrors, arranged on the other side of the protected area; after reflection, the beam passes back to the rotating mirror and from there to a receiver. This arrangement is very expensive in construction and difficult to adjust and is therefore not suitable as an alarm device for security purposes.
From West German Pat. No. 765,355, an area-security system is known in which reflective strips are arranged on both sides of the area to be monitored, and light coming from a source at one side of the area is fed, after multiple reflections on said strips, to a receiver provided on the other side of the protected area. This system is also very expensive, and it is sensitive to vibration and difficult to adjust, since accurate mirror adjustments are required.
From West German OS No. 3,134,815, an area-security system is known in which a source which emits radiation over its entire length is arranged along one side of the area to be secured, and a reflecting strip is arranged along the other side; a radiation detector is provided at each of the ends of the source of radiation and is connected to an evaluation circuit. This area-security system does not have the aforementioned disadvantages of sensitivity of adjustment and sensitivity to vibration and is simple in construction. Furthermore, it is characterized by a very dense field of radiation and accordingly by a high sensitivity of response. However, its susceptibility to false alarms makes this area-security system of questionable utility in the burglar-alarm field. In order to be able to dependably detect small objects when this area-security system is developed for penetration (intruder-alarm) security, it is necessary to select a very low sensitivity of response (about 1%). Experience has shown that, in this circumstance, local disturbances of the field, as caused for example by insects, are also detected and thus produce false alarms. Further, this known system has no provision for component failure, particularly failure of transmitting and receiving elements, which in the case of this known system leads to reduced reliability of operation of the security system, e.g., resulting from "holes" in the surface to be monitored.